1. Field of the Invention
The present invention relates to resistor networks, and more particularly, to a multilayer thick film resistor network adapted for surge protection applications.
2. Description of Related Art
Resistors are electronic devices that oppose current flow without causing any phase shift to a propagating electrical signal. Traditionally, resistors were individually packaged using axial electrical leads that permit the resistor to be electrically connected to other circuit elements, such as to a printed circuit board. In the last several years, however, it has become increasingly common to include multiple resistors within a single package. In such packages, referred to as resistor networks, the resistors may either be isolated from each other or internally connected together or to a common terminal pin. Examples of common resistor network packaging include single in-line packages (SIP) and dual in-line packages (DIP) used for through-hole or socket mounting onto a printed circuit board, or flat-pack and leadless chip configurations used for surface mounting onto a printed circuit board. Resistor networks enable manufacturers to minimize space and routing problems, reduce manufacturing cost per installed resistive function, and increase circuit board yields and reliability by reducing component counts.
In the telecommunications industry, resistors are used to provide protection to sensitive circuitry that may be exposed to electrical surges resulting from lightning and/or commercial power system disturbances. A surge is defined as a high-energy, short-duration voltage, current or power pulse. Despite the presence of protective devices in a telephone network that limit the effects of such surges, a portion of these disturbances may be impressed on the network equipment. If a telephone network is not properly protected against surges, the consequences may include permanent damage to internal network components and temporary instability to a telephone system. For this reason, surge resistors are commonly used in telephone networks as well as within television, facsimile, computers and other data processing equipment.
A surge resistor network provides a pair of resistors that respectively limit surge current into the tip and ring telecommunication terminals. The impedance value of the resistors is selected so as to not degrade the operating signal on the protected line, but must be high enough to ensure protection. In order to satisfy accepted telecommunications industry standards, such as IEC 801-5, ANSI/IEEE C62.41 and Bellcore TR-NWT-001089, the resistors must be capable of dissipating high energy for a short period of time. It is advantageous to include the pair of resistors within a single resistor network package in order to achieve better impedance matching between the resistors for noise reduction purposes, in addition to the other advantages of resistor networks described above.
There is a continuing need throughout the electronics industry to reduce device dimensions without impacting device functionality. In this regard, most sectors of the electronics industry have witnessed rapid advancements in miniaturization, particularly in the area of integrated circuits. In contrast, surge resistor networks have not benefited to the same degree as other electronic devices from such advances in miniaturization due to the need to maintain resistor surge characteristics that depend directly on device width and trace separation. Accordingly, it would be advantageous to provide a surge resistor network having a reduced physical size without compromising device capability.